Switching of degree of interpenetration and its effects on porosity of metal-Organic Frameworks (MOFs)

Aggarwal, Himanshu (2015-12)

Thesis (PhD)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: of solvent molecules from the channels. The primary objective of this work was to reinvestigate these systems in order to gain an understanding of the reasons behind loss of porosity in otherwise seemingly highly porous frameworks. The work provides a detailed account of switching of degree of interpenetration and its effects on porosity of MOFs. The first section describes a well-known doubly-interpenetrated framework, [Zn2(ndc)2(bpy)] (ndc = 2,6-naphthalenedicarboxylate and bpy = 4,4′-dipyridyl) which has been studied for loss of porosity upon activation. The zinc-based pillared-layered structure possesses minimal porosity when activated and the framework was thought to collapse upon desolvation, leading to unexpected sorption results. In the present study, it is shown that the structure does not collapse, but converts to its triply-interpenetrated analogue upon desolvation and the transformation occurs in a single-crystal to single-crystal manner under ambient conditions. A mechanism has also been proposed for the conversion and is supported by computational methods. In the second section, the work has been further extended to more robust and entirely different systems. Two known Cd(II) non-interpenetrated doubly-pillared MOFs, [Cd(tp)(4,4′-bpy)] (tp = terphthalate) and [Cd(atp)(4,4′-bpy)] (atp = 2-aminoterephthalate), have been studied for switching of degree of interpenetration. Both of these systems have been reported to form non-interpenetrated as well as doubly-interpenetrated structures. However, the possibility of inter-conversion has not been suggested. In the present study, these MOFs are shown to undergo a change in degree of interpenetration upon loss of solvent molecules from the channels. The transformation in these cases takes place at much higher temperatures as compared to the [Zn2(ndc)2(bpy)] case. In the final section of this thesis, the effect of switching of degree of interpenetration on the porosity of MOFs is demonstrated using a previously reported system, [Co2(ndc)2(bpy)], where an intermediate structure has been successfully isolated by modifying the activation conditions. This framework has also been reported to lose porosity upon desolvation. It has been found that the doubly-interpenetrated structure converts to its triply-interpenetrated form when activated at 120 °C, whereas the same material converts to an intermediate empty doubly-interpenetrated structure when activated under milder conditions. Sorption analysis using the intermediate twofold structure and the converted threefold structure shows a clear difference in the porosities of the two forms. Interestingly, all the transformations occur in single-crystal to single-crystal fashion.

AFRIKAANSE OPSOMMING: ‘n Aantal metaal-organiese raamwerke (MOFs) wat poreusheid verloor wanneer oplosmiddel molekules die kanale verlaat, is gerapporteer. Die hoofdoel van hierdie werk was om hierdie stelsels te herondersoek om ‘n beter begrip te kry van die rede vir die verlies van poreusheid in ‘n raamwerk wat andersins skynbaar hoogs poreus is. Die werk verskaf ‘n gedetaileerde verslag van verandering in die graad van interpenetrasie en die effek daarvan op die poreusheid van MOFs. Die eerste afdeling beskryf ‘n welbekende dubbel-geïnterpenitreerde raamwerk, [Zn2(ndc)2(bpy)] (ndc = 2,6-naphthalenedicarboxylate and bpy = 4,4′-dipyridyl) wat ondersoek is vir verlies van poreusheid na aktivering. Die geaktiveerde sink-gebaseerde pilaar-laag struktuur toon minimale poreusheid en daar is gemeen dat die struktuur inmekaar val wanneer die oplosmiddel molekules verwyder word. Dit het dan gelei tot onverwagse sorpsie resultate. In die huidige studie, word daar bewys dat die struktuur nie inmekaar val nie, maar omskakel na die trippel-geïnterpenitreerde vorm wanneer die oplosmiddel molekules verwyder word. Hierdie transformasie vind plaas op ‘n enkel-kristal na enkel-kristal wyse onder atmosferiese toestande. ‘n Meganisme is ook voorgestel vir die omskakeling en dit word ondersteun deur berekeningsmetodes. In die tweede afdeling is die werk verder uitgebrei na meer robuuste en totaal verskillende stelsels. Twee bekende Cd(II) nie-geïnterpenetreerde dubbel-pilaar MOFs, [Cd(tp)(4,4′-bpy)] (tp = terphthalate) and [Cd(atp)(4,4′-bpy)] (atp = 2-aminoterephthalate), is ondersoek vir omskakeling van graad van interpenitrasie. Dit is gerapporteer dat beide stelsels nie-geïnterpenitreerde, sowel as dubbel-geïnterpenitreerde strukture vorm, hoewel die moontlikheid van omkeerbare omskakeling nie oorweeg is nie. In hierdie studie word gewys dat die MOFs ‘n verandering in die graad van interpenetrasie ondergaan, wanneer oplosmiddel molekules uit die kanale verwyder word. Die transformasie, in hierdie geval, vind plaas by veel hoër temperature as in die geval van [Zn2(ndc)2(bpy)]. In die laaste afdeling van hierdie tesis word die effek van omskakeling van die graad van interpenetrasie op die poreusheid van MOFs gedemonstreer deur gebruik te maak van ‘n voorheen gerapporteerde stelsel, [Co2(ndc)2(bpy)], waar ‘n oorgangstruktuur suksesvol geïsoleer is deur middel van aanpassing van die aktiveringstoestande. Dit is ook gerapporteer dat hierdie raamwerk poreusheid verloor wanneer die oplosmiddel molekules verwyder word. Daar is bevind dat die dubbel-geïnterpenitreerde struktuur omskakel na sy trippel-geïnterpenitreerde vorm wanneer dit geaktiveer word teen 120 °C, terwyl dieselfde material omskakel na ‘n leë dubbel-geïnterpenitreerde struktuur as oorgangsproduk wanneer dit onder sagter toestande geaktiveer word. Sorpsie analise van die oorgangsproduk (tweevoudige struktuur) en die omgeskakelde drievoudige struktuur wys ‘n duidelike verskil in die poreusheid van die twee vorms. Dit is interessant dat al die transformasies op ‘n enkel-kristal na enkel-kristal wyse plaasvind.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/97708
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